One of the recognized methods of oxidizing secondary or tertiary amines comprises reacting the substrate with hydrogen peroxide, sometimes in the presence of a catalyst or other additives to promote the reaction. Thus, for example the addition of a chelating agent selected from EDTA or stannate together with carbonic acid or a carbonate has been suggested in U.S. Pat. Specification No. 4 247 480 to Nissan Chem Ind KK. Many other papers or patents also describe other variations to the reaction between amines and hydrogen peroxide.
However, there are two aspects of the hydrogen peroxide based processes to which insufficient attention has been paid hitherto, one aspect comprising the generation in situ of unwanted nitrosamine by-products and the second aspect comprising the impairment of production efficiency under non-ideal operating conditions.
In the first aspect, which in practice is often the more important aspect, the present investigations have shown that even when tertiary amines are being converted relatively efficiently to amine oxides, there is a distinct tendency for nitrosamines to be produced as a byproduct. Their presence is disadvantageous for several reasons, including especially the fact that nitrosamines are carcinogenic as a class and less importantly the typical yellow color which they can impart to the normally white product, when present in relatively high concentrations, neither disadvantage being acceptable to discriminating customers for the product. Major uses of amine oxides include personal hygiene preparations, so that it is of practical significance to either prevent the formation of nitrosamines or subsequently remove them before the amine oxide is incorporated in such formulations.
We have observed that the reaction between amines and hydrogen peroxide can result in the product being noticeably colored yellow. Some, though probably not all of the yellowing can arise from the presence of nitrosamine compounds, and some from the presence of other colored impurities. Whilst the presence of a yellowing is often an indicator that nitrosamines are present, the absence of color is no guarantee that nitrosamines too are absent, because the human eye is not a sufficiently sensitive detector to distinguish between tolerable and excessive residual levels of nitrosamines; the material can appear reasonably white to the eye, but still contain excessive residual nitrosamines. It will of course be recognized that nitrosamines are normally formed in the oxidation process of tertiary amines via a secondary amine that is present as impurity in the feedstock or is generated in situ as a result of competing side reactions. Accordingly, the problem of nitrosamine production can constitute a similar or even greater problem when the substrate is essentially a secondary amine.
The second aspect can be of importance because our current investigations have also shown that the peroxide/amine reaction is sensitive to the presence of a number of interfering substances and especially transition metals. Such materials can be present as impurities in the reactants, possibly as a result of the method or their manufacture, or introduced in water added as diluent in the reaction mixture, or can be extracted from the pipework or walls of the reaction vessels or holding tanks or resulting from inadvertant ingress of foreign bodies such as dust. The net result of such substances being present is to reduce the yield of product, and substantial loss of reaction efficiency can be encountered in cases of gross contamination. Herein, when high levels of the interfering materials are present, the conditions are sometimes described as "stress conditions".
In practical operation of the reactions involving amines and hydrogen peroxide, the interfering substances are normally present as a result of causes other than their deliberate addition, which causes are often outside the direct control of the process user, so that he cannot simply predict whether or not the substances will be present. He could analyze every sample for impurities with a view to not employing those which failed to meet an empirically determined purity standard, but that would constitute an onerous and costly task. Accordingly, it would be of practical benefit to find a means that can ameliorate any detrimental effects of the interfering substances, a means which is employable all the time and which does not interfere with the reaction under more favorable operating conditions, sometimes referred to herein as "ideal" conditions.
Moreover, the two effects can occur together, so that what is really preferable would be a treatment or combination of compatible treatments which ameliorates or overcomes both problems.